Just a work in progress.... using a breadboard.
API schematics found on the Internet... just google API 312 schematic. You can see the printout of the schematic on the photo.
I'm using a Melcor Discrete opamp instead of the original 2520 opamp (which you can't buy unless you have a broken 2520 to surrender to API). Although there are many substitute opamps you can use like the Jensen 990C, Forsell Technologies 992, John Hardy opamp, etc... or if you're really scraping, just use an NE5534 IC chip.